1. Field of the Invention
This invention relates to improvements in circuitry for kelvin current sensing. More particularly, the invention is drawn to voltage sensing internally within the semiconductor device to reduce the effects of any parasitic resistances while sensing the voltages.
2. Description of the Related Art
Although the invention pertains to polyphase DC motors, in general, it finds particular application with three-phase DC motors. These motors can be of the brushless, sensorless type that are used for rotating data media, such as the motors in hard disk drives, CD ROM drives, floppy disks and other computer related applications.
These motors are typically thought of as having a stator with three coils connected in a "Y" configuration, although in actual systems many stator coils may be used with multiple motor poles. In operation, the coils are selectively energized to set up a current path through two coils of the "Y" configuration and to leave a third coil floating. Another method is to energize sequentially one coil at a time by having a current path through a single coil to a center tap. In either case, a sequence of energized coils is arranged so as the current paths are changed, or commutated, at least one of the coils used to form the current path becomes a floating coil in the next sequence.
For a three phase dc motor, there are typically six driver transistors that may be, for example, CMOS transistors, that control the current applied to three stator coils. Three upper driver transistors connect power to selected coils, and three lower driver transistors connect selected coils to ground. Typically, all six driver transistors are contained on a single semiconductor integrated circuit chip. Each of the lower driver transistors are connected to an output connection of the semiconductor device, with each of the output connections being connected to the upstream side of a sense resistor, R.sub.sense, the downstream side of which being attached to ground.
To control the lower driver transistors, a comparator is provided as a part of the circuitry of the integrated circuit chip. The comparator has a noninverting input connected to a bonding pad on the semiconductor chip that is connected to an external connection pin. A reference voltage, V.sub.in, is supplied to the external connection pin by an outside source. Alternatively, the reference voltage may be developed internally to the device by other circuitry (not shown). The inverting input of the comparator is connected to a second bonding pad on the semiconductor chip that is connected to a second external connection pin. This second external connection pin is connected to the sense resistor, R.sub.sense, in the manner described above. A comparison is then made between the voltages appearing on these two external connection pins, this comparison being referred to herein as "kelvin current sensing."
Thus, kelvin current sensing monitors the voltage changes across the sense resistor, R.sub.sense, and controls the transistors that drive the stator windings in response to that voltage change. If the voltage across R.sub.sense becomes too large, for instance, higher than V.sub.in, then the output of the comparator changes to cause all of the lower driver transistors to turn off.
In a typical brushless, sensorless DC motor, a sequencer is used to create signals to control the current in the various stator coils. Each coil is connected to a pin of a semiconductor device that contains an integrated circuit chip that contains the driver and control circuitry, and, in some cases, the sequencer circuit. For example, in FIG. 1, a schematic diagram of a portion of a prior art driver circuit, the stator coils 11, 12, and 13 of a three phase DC motor are shown, connected respectively to nodes A, B, and C. The sequencer circuit (not shown) supplies six sequencer signals, UA, UB, UC, LA, LB, and LC, each connected to control, directly or indirectly, one of the driver transistors. Thus, upper driver transistors 20, 21, and 22 are directly controlled by UA, UB, and UC, respectively. When any of these sequencing signals are high, the corresponding driver transistor turns on to connect the associated stator coil to V.sub.cc.
The lower driver transistors 23, 24, and 25 are indirectly controlled by signals LA, LB, and LC. Each of the gates of the lower driver transistors 23, 24, and 25 is connected to a respective switch 26, 27, and 28. Each switch has two states. The first state is ground and the second state is the output of an operational amplifier 29. If desired, a PWM switch could be substituted for these three switches 26, 27, and 28 to control the lower driver transistors.
The lower driver transistors 23, 24, and 25 have their sources connected to the pins 30, 31, and 32, respectively. (The various pins shown in FIG. 1 represent the bonding pads, bonding wires, and external package pins, and are shown in abbreviated form for simplicity.) The pins 30, 31, and 32 are connected externally from the device, and to the upstream side of a resistor 16 (R.sub.sense). The source terminals of the lower driver transistors 23, 24, and 25 are often not internally connected because a major metal interconnection would be necessary.
The downstream side of the resistor 16 is connected to ground. At a point near the resistor 16, a connection 35 is made between the resistor 16 and a "sense" pin 33. The pin 33 has an internal connection to the inverting input of operational amplifier 29. The noninverting input of the operational amplifier 29 is connected to a pin 34, to which a voltage signal V.sub.in from an external voltage source may be connected.
The output of the operational amplifier 29 is used to control the gates of the lower driver transistors 23, 24, and 25 in normal operation. Thus, when one of the lower sequencer signals LA, LB or LC is a logic high, the output of the operational amplifier 29 is connected by the respectively associated switch 26, 27, or 28 to the gate of the corresponding lower driver transistor to turn on that driver transistor. At the same time, the sequencer also sends a signal UA, UB or UC to turn on a corresponding one of the upper driver transistors. Therefore, current flows from V.sub.cc through the selected upper driver transistor, through two of the coils in the stator, through the selected lower driver transistor, and then through R.sub.sense to ground.
As the current is flowing through R.sub.sense, the voltage may change due to fluctuations of current in the various circuit switches. If the voltage becomes larger than the voltage signal V.sub.in, then the comparator 29 will change state and reduce conduction or turn off all of the lower driver transistors.
These structures and methods have a disadvantage in that an extra pin is needed to connect the R.sub.sense to the inverting input of the comparator.